Hereinafter, the moving object shall be denoted “card”, it being understood that this designation does not in a limiting way designate a particular form of object, and that it applies in particular to a radiofrequency identification tag normally designated “RFID tag”.
In these systems, the reader generally sends a radiofrequency carrier, periodically modulated to form a radiofrequency interrogation signal; a typical frequency is 13.56 MHz, amplitude or frequency modulated, and it is this frequency that will be mentioned in the examples that follow; the reader then waits for a response; if a card likely to communicate with the reader is present in a determined geographic field near to the reader, the card responds and dialogues with the reader. The response can be the transmission of a simple digital identification, or can even be more complex. The card can be without any independent power source, and in this case the power that enables it to send its response is supplied inductively by the reader; the power can even be supplied by the interrogation signal itself for very short distance communication applications (a few centimetres to a few tens of centimetres, even a few metres).
These communication systems can be used in particular for secure identification applications and the confidential content of the dialogue between the card and the reader must be protected against possible intrusions by malicious third parties. It is in particular essential to prevent a third party from accessing the data sent by the card, because the observation, then the copying, of data transmitted by an authentic card could be used to then deceive the system with a false card.
Such protection is obtained mainly by the use of methods of encrypting the response from the card: the card sends an encrypted response instead of sending a plain language response, and only the reader can in theory decrypt the response. The encryption methods are, however, not 100% safe; if the stakes are high, cheats can manage to determine the encryption keys by picking up and analysing the radiofrequency signals transmitted in the communication field between the card and the reader.
To enhance security, patent application WO2006035178 proposes that the reader transmit a scrambling electromagnetic noise in the useful field where the card is located, at the same time as the card transmits its response; the electromagnetic noise then prevents a third party from detecting the response transmitted by the card. The noise is sent in the frequency spectral band of the useful signal transmitted by the card, and it has particular characteristics, that the reader knows, such that the reader can subtract it from the received radiofrequency signal. The subtraction is not, however, easy because of the fact that the injection of noise acts as a modulation of the carrier of the communication signal which is transmitted by the reader to the card, this noise modulation being superimposed on a useful modulation that is necessary for the communication to the card.